Ok, if its a front wired SRC you are going to have to open the gun up, have a look and see how the wires (not the motor ones) leave the gearbox, do they leave from the front or the back?
If they leave from the back then half your job is done, all you have to do then is just rewire it to the rear by not feeding it to the front, close the gun, power it up and test it.
If the wires leave the gearbox from the front however you are going to need to open the gearbox, take out the trigger contacts, desolder the wires from the contacts, re-position them on the contacts and solder them back on, close the gearbox, put the gun back together and test it.